You know who else started selling oil in currencies other than the US dollar? Iraq, in 2000.

Hmm, what a coincidence.

In case anyone’s missing the significance: the only thing propping up the value of the dollar is that everyone needs oil, and you need dollars to buy oil. If countries could buy all their oil in (say) Euros, they’d stop dealing in dollars, stop lending the US money to run the deficit, and make it impossible to run the US economy as it’s currently being run. Or as the Economist puts it:

A shift towards a looser peg in the GCC would undoubtedly hurt the greenback. At the very least, dollars would be purchased at a slower rate—leading to what Mr Lyons calls “passive diversification”. At worst, the policy might encourage others to follow, sparking panic sales of American assets.

i.e. a very real chance of the US economy entering another Great Depression.